A terahertz wave has major characteristics as listed below.
Firstly, since the wavelength of a terahertz wave is relatively short, it is transmitted through a non-metal substance just like X-rays. Secondly, there are many biomolecules and medicines having a characteristic absorption spectrum in the frequency band of terahertz wave. Thirdly, it has spatial resolving power suited for various imaging applications because the pulse width in time domain is relatively short.
Fields of application of terahertz waves having characteristics as listed above include spectral analysis techniques for the inside of substance, safe see-through imaging apparatus that can replace X-ray fluoroscopic apparatus and techniques for analyzing biomolecules.
Japanese Patent No. 3387721 discloses that THz-TDS (terahertz time domain spectroscopy) can suitably be used as spectroscopy for employing a terahertz wave. FIG. 10 of the accompanying drawings schematically illustrates an apparatus realized by applying terahertz time-domain spectroscopy.
Referring to FIG. 10, femtosecond light pulse L1 emitted from a femtosecond pulse light source 901 that is a titanium-sapphire laser is split to produce two light pulses L2 and L3 by a beam splitter 902. The light pulse L2 excites terahertz pulse generator 903 to generate a terahertz pulse T1. On the other hand, the light pulse L3 is optically delayed by a light pulse delaying section 907 that is formed by using a combination of several planar reflectors and a moving mirror. The optically delayed light pulse L4 is made to enter a terahertz pulse detector 906 by way of a mirror 908.
A photoconductive antenna formed by depositing a dipole antenna on an LT-GaAs thin film that is grown on a GaAs substrate at low temperatures can suitably be employed for both the terahertz pulse generator 903 and the terahertz pulse detector 906. Excited carriers are accelerated to generate a terahertz pulse T1 in the generator 903 as a femtosecond laser pulse L2 is driven to strike the gap of the dipole antenna as pump light in a state where a voltage is applied to the gap. On the other hand, as a femtosecond laser pulse L4 is driven to strike the gap as probe light in the detector 906, an electric current signal that is proportional to the electric field of the terahertz pulse at the time when probe light strikes the gap is generated.
The terahertz pulse T1 generated by the terahertz pulse generator 903 is focused by a focusing optical system 904. An object of examination 100 is arranged at the focus position. The terahertz pulse T2 transmitted through the object of examination 100 is made to enter the terahertz pulse detector 906 by the focusing optical system 905. As pointed out above, an electric current signal that is proportional to the electric field of the terahertz pulse T2 at the time when probe light L4 strikes the gap is generated in the terahertz pulse detector 906. This signal is amplified by an amplifier 909 and then taken into adjusting section 910 that is formed by a signal processing circuit, a personal computer and so on.
The time waveform of the terahertz pulse T2 can be acquired by sequentially changing the optical delay by means of the light pulse delaying section 907 and observing the terahertz pulse, changing the timing of detection. Spectral information (amplitude information on each frequency component, intensity information on each frequency (wavelength) etc. can be obtained by Fourier-transforming the time waveform.
A two-dimensional image of the object of examination 100 can be obtained by scanning the object of examination 100 on a plane perpendicular to the optical axis by means of a scanning stage 912. Then, a so-called spectral image that provides spectral information on each pixel of the two-dimensional image can be obtained by observing the time waveform of the terahertz pulse on a pixel by pixel basis by using the same technique. Such an image can be displayed on a display section 911.